Until recently if you took milk, and removed the cream you could not sell it as "skim milk" unless you added vitamins A and D. You had to sell your pure milk with cream removed as, get this, "imitation milk product".
Velveeta, which this stuff resembles at least visually, is not legally considered cheese because it contains milk protein concentrate which is made from, you guessed it, milk.
Which is also what you make cheese from.
Go figure.
When you skim off the fat you're also skimming off the fat soluble vitamins like A and D. So in a sense it's now nutritionally less than milk, but to say it's an imitation isn't quite accurate. If you then fortify it by adding back the vitamins it becomes more on par with whole milk nutritionally, but since those vitamins are an artificial addition you could almost make the case that it's now more of an imitation than the unfortified skim milk is.
No doubt the intent was to discourage people from buying skim milk by giving it an off-putting name like "imitation milk product," thereby driving them towards the fortified skim milk, with the ultimate goal of avoiding an unwitting shortfall of those vitamins in your (or your kids') diet.
A similar example that comes to mind is white flour, except here the Gov was more direct in its labeling requirements. When you strip the germ from whole wheat flour you lose the vitamins. Flour producers then add the vitamins back in and call it fortified white flour--which is really what it is.